1. Field of the Invention
The present invention relates to an exhaust gas cleaning device of an internal combustion engine, which has a particulate filter for capturing particulate matter contained in exhaust gas exhausted from the internal combustion engine.
2. Description of Related Art
One previously proposed exhaust gas cleaning device of an internal combustion engine provided in, for example, a vehicle, has a particulate filter, which is arranged in an exhaust gas passage of the internal combustion engine to capture particulate matter contained in exhaust gas exhausted from the internal combustion engine.
In the particulate filter, supplied exhaust gas is passed through porous walls. At this time, the particulate matter contained in the exhaust gas is captured by surfaces and minute holes of the porous walls. When the amount of accumulated particulate matter in the particulate filter becomes excessive, flow resistance in the particulate filter increases. This causes an increase in a back pressure of the internal combustion engine to cause a decrease in an output power of the engine. Thus, in such a case, the particulate filter needs to be regenerated by removing the accumulated particulate matter from the particulate filter and thereby recovering the exhaust gas flow conducting performance of the particulate filter. In general, a state of accumulation of the particulate matter in the particulate filter is monitored. When the amount of accumulated particulate matter in the particulate filter exceeds a predetermined value, the temperature of the particulate filter is increased by, for example, post fuel injection to combust the accumulated particulate matter and thereby to forcefully regenerate the particulate filter.
The amount of exhausted particulate matter, which is exhausted from the internal combustion engine, is regarded as the amount of collected particulate matter, which is collected by the particulate filter. This amount of collected particulate matter is cumulated to obtain a current estimated value of the amount of accumulated particulate matter. At this time, as recited in Japanese Unexamined Patent Publication No. 1-318715 (corresponding to U.S. Pat. No. 4,902,487), the particulate matter, which is accumulated in the particulate filter, is combusted (oxidized) at a relatively low temperature with aid of NOx contained in the exhaust gas. Thus, when the amount of accumulated particulate matter needs to be estimated with a relatively high precision, a decrease in the amount of accumulated particulate matter, which is caused by the oxidizing reaction, needs to be considered.
For example, according to Japanese Unexamined Patent Publication Number 2001-280118, the particulate filter is regenerated by the oxidizing reaction of the accumulated particulate mater with NOx. In this case, there is computed a difference between an estimated value of the amount of exhausted particulate matter, which is estimated based on an operational state of the internal combustion engine, and an estimated value of the amount of combusted particulate matter, which is estimated based on the temperature of exhaust gas that passes through the particulate filter. Then, this difference is cumulated to obtain a corresponding current estimated value of the amount of accumulated particulate matter.
Japanese Unexamined Patent Publication No. 1-318715 and Japanese Unexamined Patent Publication Number 2001-280118 recite the methods of continuously regenerating the particulate filter through the oxidizing reaction of the accumulated particulate matter with the NOx contained in the exhaust gas, which is supplied to the particulate filter. In the continuous regeneration, normally, a catalyst is positioned in an upstream part of the particulate filter to promote the oxidizing reaction of the accumulated particulate matter with the NOx. However, in the particulate filter of the typical forceful regeneration type, the accumulated particulate matter is also removed through the oxidizing reaction of the accumulated particulate matter with the NOx contained in the exhaust gas, which is supplied to the particulate filter. Furthermore, in the patent document 2, the typical forceful regeneration type is used in view of the fact that the oxidizing reaction between the exhausted particulate matter and NOx does not proceeds sufficiently when the temperature is too low. Thus, when the computed value of the amount of accumulated particulate matter exceeds the predetermined value, the temperature of the exhaust gas is forcefully increased to remove the accumulated particulate matter.
However, at the time of computing the current amount of accumulated particulate matter, when the current amount of accumulated particulate matter is computed upon consideration of only the oxidizing reaction of NOx as recited in Japanese Unexamined Patent Publication No. 1-318715, a value of the amount of combusted particulate matter could be possibly underestimated, so that an accuracy of an estimated value of the amount of accumulated particulate matter may be deteriorated. In Japanese Unexamined Patent Publication No. 1-318715, there is recited only the oxidizing reaction of the particulate matter with NOx contained in the exhaust gas, which is supplied to the particulate filter. However, in reality, a portion of NOx contained in the exhaust gas, which is previously supplied to the particulate filter, is adsorbed in the accumulated particulate matter. The accumulated particulate matter is reduced through the reaction of the accumulated particulate matter with the portion of NOx, which is adsorbed in the accumulated particulate matter. This is the reason for deteriorating the accuracy of the estimated value of the amount of accumulated particulate matter. Because of this, the estimated value of the amount of accumulated particulate matter becomes greater than the actual value of the amount of accumulated particulate matter. As a result, a frequency of regeneration of the particulate filter is increased, and fuel consumption is significantly deteriorated.
As described above, the prior art method is not sufficiently accurate for monitoring the combustion state of the accumulated particulate matter, and therefore the amount of accumulated particulate matter cannot be correctly measured.